package com.zw.cloud.mybatis.plus.util;

import java.math.BigDecimal;

public class PI {
    public static void main(String args[]) throws NumberFormatException {
        int digits = 100000; // 计算1000位
        long begin = System.currentTimeMillis();
        String pi = computePi(digits).toString();
        long end = System.currentTimeMillis();
        System.out.println("pi" + pi);
        System.out.println((end - begin) + "ms");
    }

    /** constants used in pi computation */
    private static final BigDecimal FOUR = BigDecimal.valueOf(4);

    /** rounding mode to use during pi computation */
    private static final int roundingMode = BigDecimal.ROUND_HALF_EVEN;

    /**
     * Compute the value of pi to the specified number of digits after the
     * decimal point. The value is computed using Machin's formula:
     *
     * pi/4 = 4*arctan(1/5) - arctan(1/239)
     *
     * and a power series expansion of arctan(x) to sufficient precision.
     */
    public static BigDecimal computePi(int digits) {
        int scale = digits + 5;
        BigDecimal arctan1_5 = arctan(5, scale);
        BigDecimal arctan1_239 = arctan(239, scale);
        BigDecimal pi = arctan1_5.multiply(FOUR).subtract(arctan1_239)
                .multiply(FOUR);
        return pi.setScale(digits, BigDecimal.ROUND_HALF_UP);
    }

    /**
     * Compute the value, in radians, of the arctangent of the inverse of the
     * supplied integer to the specified number of digits after the decimal
     * point. The value is computed using the power series expansion for the arc
     * tangent:
     *
     * arctan(x) = x - (x^3)/3 + (x^5)/5 - (x^7)/7 + (x^9)/9 ...
     */
    public static BigDecimal arctan(int inverseX, int scale) {
        BigDecimal result, numer, term;
        BigDecimal invX = BigDecimal.valueOf(inverseX);
        BigDecimal invX2 = BigDecimal.valueOf(inverseX * inverseX);

        numer = BigDecimal.ONE.divide(invX, scale, roundingMode);

        result = numer;
        int i = 1;
        do {
            numer = numer.divide(invX2, scale, roundingMode);
            int denom = 2 * i + 1;
            term = numer.divide(BigDecimal.valueOf(denom), scale, roundingMode);
            if ((i % 2) != 0) {
                result = result.subtract(term);
            } else {
                result = result.add(term);
            }
            i++;
        } while (term.compareTo(BigDecimal.ZERO) != 0);
        return result;
    }
}